With ever-increasing demand for energy savings and emission reduction, more and more vehicle parts, such as automotive vehicle parts, are now being manufactured using high strength steel sheets, which are stronger and can be made thinner to reduce the vehicle mass and thus improve vehicle fuel efficiency. Increasing importance is also being placed on vehicle safety to protect a driver and passengers upon collision.
Generally, steel sheets having a high strength exhibit a high impact resistance, and thus are also more favorable. However, a problem arises in that an increase in strength of a steel sheet generally decreases its formability, and thus using such a sheet to manufacture complicated parts becomes more difficult.
A known solution to this problem is dual phase steel, which possesses microstructures of martensite islands embedded in a ferrite matrix. Due to a superior combination of high tensile strength, high elongation, continuous yielding, low yield ratio and high work hardening, dual phase steel is not only strong, but also has good formability, such as press-forming and draw-forming properties, and exhibits high crash resistance. Applications of dual phase steel sheets in the vehicle industry can thus help to improve vehicle fuel efficiency and durability, and further improve the safety of passengers.
The previous research and development in the dual phase steel sheet field have resulted in a number of methods for producing cold rolled, hot-dip coated dual phase steel sheets, many of which are summarized and reviewed below.
U.S. Published Patent Application No. 2005/0247383 A1 to Utsumi, et al. discloses a hot-dip galvanized dual phase steel sheet. The said steel sheet comprises, by weight %, 0.05 to 0.12% carbon, not more than 0.05% silicon, 2.7 to 3.5% manganese, 0.2 to 0.5% chromium, 0.2 to 0.5% molybdenum, not more than 0.10% aluminum, not more than 0.03% phosphorus, and not more than 0.03% sulfur. The steel sheet is obtained by a soaking process in which the temperature is set to a range from 820 to 900° C., and the time is not less than 30 seconds.
U.S. Published Patent Application Nos. 2005/0019601 A1, 2005/0016644 A1, 2004/0108024 A1 and 2004/0007297 A1, as well as U.S. Pat. No. 6,818,074, No. 6,814,819 and No. 6,676,774, all to Matsuoka et al., relate to a high ductility steel sheet containing appropriate amounts of carbon, silicon, manganese, phosphorus, sulfur, aluminum, nitrogen, and 0.5 to 3.0% copper. A composite structure of the said steel sheet has a ferrite phase or a ferrite phase and a tempered martensite phase as a primary phase, and a secondary phase containing retained austenite in a volume ratio of not less than 2%.
U.S. Published Patent Application No. 2004/0238082 A1 to Hasegawa, et al. discloses a high strength cold rolled dual phase steel plate. The steel consists essentially of, by weight %, 0.04 to 0.10% carbon, 0.5 to 1.5% silicon, 1.8 to 3% manganese, not more than 0.02% phosphorus, not more than 0.01% sulfur, 0.01 to 0.1% aluminum, not more than 0.005% nitrogen, and the balance being iron and inevitable impurities. The steel sheet has ductility with an elongation of 18% or more, stretch flangeability with a hole expansion ratio of 60% or more and a tensile strength of 780 MPa or more.
U.S. Published Patent Application No. 2004/0238081 A1 to Yoshinaga, et al. describes a steel sheet excellent in workability, including, by weight %, 0.08 to 0.25% carbon, 0.001 to 1.5% silicon, 0.01 to 2% manganese, 0.001 to 0.06% phosphorus, not more than 0.05% sulfur, 0.001 to 0.007% nitrogen, 0.008 to 0.2% aluminum, and at least 0.01% iron. The steel sheet has an average r-value of at least 1.2, an r-value in the rolling direction of at least 1.3, an r-value in the direction of 45 degrees to the rolling direction of at least 0.9, and an r-value in the direction of a right angle to the rolling direction of at least 1.2.
U.S. Published Patent Application No. 2004/0238080 A1 to Vandeputte, et al. relates to a cold rolled, possibly hot dip galvanized steel sheet with thickness lower than 1 mm, and tensile strength between 800 MPa and 1600 MPa, while the A80 elongation is between 5 and 17%. The composition of the steel is characterized by, in weight %, 0.10 to 0.25% carbon, 0.15 to 0.3% silicon, 1.2 to 2% manganese, 0.01 to 0.06% phosphorus, not more than 0.005% sulfur, not more than 0.01% nitrogen, not more than 0.1% aluminum, 0.001 to 0.0035% boron, not more than 0.04% Tifactor (Tifactor=Ti-3.42 N+10), 0.02 to 0.08% Niobium, 0.25 to 0.75 chromium, 0.1 to 0.25 molybdenum, not more than 0.005% calcium, and the remainder being substantially iron and incidental impurities.
U.S. Published Patent Application No. 2004/0211495 A1 and U.S. Pat. No. 6,811,624, both to Hoydick, as well as U.S. Pat. No. 6,312,536 to Omiya, disclose a hot dip galvanized dual phase steel sheet. The steel has the composition of, in weight %, 0.02 to 0.20% carbon, 0.010 to 0.150% aluminum, not more than 0.01% titanium, not more than 0.5% silicon, not more than 0.06% phosphorus, not more than 0.030% sulfur, 0.8 to 2.4% manganese, 0.03 to 1.5% chromium, and 0.03 to 1.5% molybdenum.
U.S. Published Patent Application No. 2004/0047756 A1 to Rege, et al. relates to a method of producing cold rolled and annealed dual phase high strength steel sheets, including hot dip galvanized and galvannealed steel sheets having a tensile strength of at least about 750 MPa. Rege, et al. disclose that the effect on hardenability of chromium and vanadium enables production of a high strength product having a low yield ratio.
U.S. Published Patent Application No. 2004/0035500 A1 to Ikeda, et al. provides a dual phase steel sheet with good bank-hardening properties. The steel is characterized in containing, in mass %, 0.06 to 0.25% carbon, 0.5 to 3% silicon plus aluminum, 0.5 to 3% manganese, not more than 0.15% phosphorus, not more than 0.02% sulfur; and also meeting the conditions that retained austenite is at least 3%, bainite is at least 30%, and ferrite is no more than 50%.
U.S. Published Patent Application No. 2003/0221752 A1 and U.S. Pat. No. 6,709,535, both to Utsumi et al., are relevant to a dual phase steel sheet containing, by weight %, 0.08 to 0.20% carbon, not more than 0.5% silicon, not more than 3.0% manganese, not more than 0.02% phosphorus, not more than 0.02% sulfur, 0.001 to 0.15% aluminum, and further containing 0.05 to 1.5% molybdenum and 0.05 to 1.5% chromium.
U.S. Published Patent Application No. 2003/0084966 A1 to Ikeda, et al. discloses a dual phase steel sheet having low yield ratio, excellent in the balance for strength-elongation and for strength-stretch flange formability, and also excellent in bake hardening property containing, in weight %, 0.01 to 0.20% carbon, not more than 0.5% silicon, 0.5 to 3% manganese, not more than 0.06% aluminum, not more than 0.15% phosphorus, and not more than 0.02% sulfur. The matrix phase contains tempered martensite, tempered martensite and ferrite, tempered bainite, or tempered bainite and ferrite.
U.S. Pat. No. 6,869,691 to Nagataki, et al. is directed to a high strength hot dip galvanized steel sheet consisting essentially of, in weight %, 0.03 to 0.25% carbon, not more than 0.7% silicon, 1.5 to 3.5% manganese, not more than 0.05% phosphorus, not more than 0.01% sulfur, 0.05 to 1.0% chromium, 0.005 to 0.1% niobium, and the balance being iron.
U.S. Pat. No. 6,673,171 to Hlady, et al. is directed to a medium carbon steel sheet with enhanced uniform elongation for deep drawing applications. In one embodiment, a steel slab containing, in weight %, 0.30 to 0.70% carbon, 0.75 to 2.0% manganese, not more than 1.0% silicon, 0.020 to 0.10% aluminum, and the balance iron and incidental impurities is hot rolled to strip at a finishing temperature within the range of 839° C. (1542° F.) to 773° C. (1424° F.) and spheroidize annealed at a temperature below the A. sub.1 temperature. In a second embodiment, a steel slab containing, in weight %, 0.40 to 0.70% carbon, 0.50 to 1.50% manganese, not more than 1.0% silicon, 0.020 to 0.10% aluminum, and the balance being iron and incidental impurities, is hot rolled, cold rolled and spheroidize annealed, with various combinations of manganese and silicon within the above ranges providing lower yield strength at levels of 60 ksi, 70 ksi, and 80 ksi with a minimum 14% uniform elongation.
U.S. Pat. No. 6,641,931 to Claessens, et al. provides a method of producing a cold rolled metal coated multi-phase steel, characterized by a tensile strength of at least 500 MPa, a yield ratio lower than 0.65 in skinned conditions, lower than 0.60 in unskinned conditions, and with good metal coating adhesion behavior. The hot metal coated steel product having a steel composition, in weight %, of not more than 1.5% manganese, 0.2 to 0.5% chromium and 0.1 to 0.25% molybdenum, undergoes a thermal treatment in the hot dip metal coating line defined by a soaking temperature between Ac1 and Ac3, a primary cooling speed higher than 25° C./s and a secondary cooling speed higher than 4° C./s.
U.S. Pat. No. 6,537,394 to Osawa, et al. is related to a method for producing hot dip galvanized steel sheet having high strength. The steel sheet contains, in weight %, 0.01 to 0.20% carbon, not more than 1.0% silicon, 1.5 to 3.0% manganese, not more than 0.10% phosphorus, not more than 0.05% sulfur, not more than 0.10% aluminum, not more than 0.010% nitrogen, 0.010 to 1.0% in total of at least one element selected from the group consisting of titanium, niobium and vanadium, and the balance being iron and incidental impurities. The steel sheet has a metal structure in which the area rate of ferrite phase is 50% or more, and the ferrite phase has an average grain diameter of 10 μm or less.
U.S. Pat. No. 6,440,584 to Nagataki, et al. is directed to a hot dip galvanized steel sheet, which contains, by weight %, 0.04 to 0.12% carbon, not more than 0.5% silicon, 1.0 to 2.0% manganese, not more than 0.05% phosphorus, not more than 0.005% sulfur, 0.05 to 1.0% chromium, 0.005 to 0.2% vanadium, not more than 0.10% aluminum, and not more than 0.010% nitrogen.
U.S. Pat. No. 6,423,426 to Kobayashi, et al. relates to a high tensile hot dip zinc coated steel plate having a composition comprising, in weight %, 0.05 to 0.20% carbon, 0.3 to 1.8% silicon, 1.0 to 3.0% manganese and iron as the balance. The steel is subjected to a primary step of primary heat treatment and subsequent rapid cooling to Ms point or lower, a secondary step of secondary heat treatment and subsequent rapid cooling, and a tertiary step of galvanizing treatment and rapid cooling, so as to obtain 20% or more by volume of tempered martensite, 2% or more by volume of retained austenite, ferrite and a low-temperature transformation phase in the steel structure.
U.S. Pat. No. 6,210,496 to Takagi, et al. discloses a high strength high workability cold rolled steel plate. The steel includes, by mass %, 0.05 to 0.40% carbon, 1.0 to 3.0% silicon, 0.6 to 3.0% manganese, 0.02 to 1.5% chromium, 0.010 to 0.20% phosphorus, and 0.01 to 0.3% aluminum, with the remainder consisting essentially of iron.
U.S. Pat. No. 5,470,403 to Yoshinaga is directed to a cold rolled steel sheet and a hot dip zinc-coated cold rolled steel sheet excellent in paint bake hardenability, non-aging properties and formability, and a process for producing the same. The steel sheet consists essentially of, in weight %, 0.0005 to 0.0070% carbon, 0.001 to 0.8% silicon, 0.3 to 4.0% manganese, 0.003 to 0.15% phosphorus, 0.0005 to 0.015% sulfur, 0.005 to 0.20% aluminum, 0.0003 to 0.0060% nitrogen, not more than 0.0030% boron, where the boron satisfies that the ratio of boron/nitrogen is not larger than 1.5, and balance iron and unavoidable impurities. The steel sheet has phases transformed at low temperature in an amount greater than 5%.
U.S. Pat. No. 5,328,528 to Chen provides a process for manufacturing cold rolled steel sheets with high strength and high ductility. The steel sheets contain, in weight %, 0.08 to 0.25% carbon, 0.03 to 2.0% silicon, 0.6 to 1.8% manganese, 0.01 to 0.10% niobium, 0.01 to 0.08% aluminum, with the rest being substantially iron and unnoticed impurities.
U.S. Pat. No. 4,770,719 to Hashiguchi, et al. provides a method of manufacturing a high strength steel sheet by annealing the steel sheet after cold rolling. The steel sheet contains, in weight %, 0.03 to 0.15% phosphorus and specified amounts of carbon, manganese and aluminum as basic components and optionally contains, as a selective component, at least one element selected from a group of silicon, chromium, molybdenum and boron, and a group of niobium, titanium and vanadium.
U.S. Pat. No. 4,708,748 to Satoh, et al. discloses a method of making cold rolled dual phase structure steel sheet, which consists of, in weight %, 0.001 to 0.008% carbon, not more than 1.0% silicon, 0.05 to 1.8% manganese, not more than 0.15% phosphorus, 0.01 to 0.10% aluminum, 0.002 to 0.050% niobium and 0.0005 to 0.0050% boron. The steel sheet is manufactured by hot and cold rolling a steel slab with the above chemical composition and continuously annealing the resulting steel sheet in such a manner that the steel sheet is heated and soaked at a temperature from Ac1 to 1000° C. and then cooled at an average rate of not less than 0.5° C./s but less than 20° C./s in a temperature range of from the soaking temperature to 750° C., and subsequently at an average cooling rate of not less than 20° C./s in a temperature rage of from 750° C. to not more than 300° C.
U.S. Pat. No. 4,609,410 to Hu relates to a high strength deep drawable dual phase steel sheet, which is produced by (i) initially annealing the sheet to achieve crystallographic textures yielding high deep drawability, (ii) heating the sheet to a temperature above A1 for a time sufficient to produce from 2 to 10% austenite, and thereafter (iii) rapidly cooling to transform at least a portion of the austenite to martensite or bainite.
U.S. Pat. No. 4,436,561 to Takahashi, et al. discloses a press-formable, high strength, dual phase structure cold rolled steel sheet. The said steel sheet is made from steel consisting of, in weight %, 0.02 to 0.20% carbon, not more than 0.1% silicon, 1.0 to 2.0% manganese, 0.005 to 0.10% acid-soluble aluminum, and 0.0003 to 0.0050% boron.
U.S. Pat. No. 4,398,970 to Marder, et al. is directed to a method to make and the resulting product of titanium and vanadium dual phase steel. The method includes the steps of (i) preparing an aluminum-killed steel consisting essentially of, in weight %, 0.05 to 0.15% carbon, not more than 2.0% manganese, not more than 1.0% silicon, 0.03 to 0.15% vanadium, and a sufficient amount of titanium, with the balance essentially being iron, where the titanium addition should be at least equal to the atomic percent of the sulfur plus nitrogen, but no more than about 1.6 times; (ii) intercritically annealing such steel within the alpha.+gamma. temperature range and (iii) cooling to room temperature.
U.S. Pat. No. 4,376,661 to Takechi, et al. discloses a method of producing a dual phase structure cold rolled steel sheet, which contains, in weight %, 0.01 to 0.05% carbon, not more than 0.2% silicon, 1.7 to 2.5% manganese, 0.01 to 0.10% aluminum, with the balance being iron and unavoidable impurities. The method comprises hot rolling and cold rolling by conventional process, holding the produced steel sheet for 20 seconds to 20 minutes at a temperature ranging from 720 to 850° C., and cooling the steel sheet at a cooling speed between 3° C./s and 50° C./s and also having a value (C./s) shown by following formulae:12.times.[Mn(%)].sup.2−62.times.[Mn(%)+8].
The disclosures of all patents and published patent applications, mentioned here, are incorporated by reference.
As disclosed by many of the patents and/or published patent applications reviewed above, carbon and/or manganese are elements often added in high concentrations into steel sheets in order to obtain high hardenability and strength. However, when the concentrations of these elements are too high, the formability and weldability of manufactured steel sheets could be adversely affected.
Some of the above-noted patents and/or published patent applications describe employing a relatively high amount of copper as an alloy in the steel to achieve a desired hardenability and strength. However, this alloy is expensive, and its presence could deteriorate the surface quality and weldability of the steel sheets.
Some of the above patents and/or published patent applications describe employing phosphorus as a major strengthening element. When phosphorus is near the upper limit as described in these patents and published patent applications, the segregation of phosphorus at grain boundaries could occur, which results in brittleness of the steel sheet, and in turn impairs its formability and fatigue property. When too much phosphorus is added, the spring back angle of parts formed from the steel sheet could also be increased. In other words, the shape-fixability of the steel sheet becomes worse. Regarding the manufacturing processes, the castability and rollability of the steel sheet are also deteriorated when too much phosphorus is added. Moreover, a high phosphorus concentration in steel sheets could adversely affect coating adhesion during the hot dip coating processing.
Boron is another element described in some of the above patents and/or published patent applications as being employed for improving the hardenability and strength of the steel sheet. However, when boron is added in excess, the rollability of the steel sheet is significantly lowered. Also, the segregation of boron at grain boundaries deteriorates the formability and weldability of the steel sheet.
Vanadium, niobium and titanium are elements which are described in some of the above patents and/or published patent applications. These elements may be used alone or may be employed in combination. When concentrations of these elements are relatively high, the respective carbides, nitrides or complex precipitates are formed in the steel sheet, resulting in so-called precipitation hardening. Then, such precipitates can not only markedly reduce castability and rollability during manufacturing the steel sheet, but also can deteriorate the formability of the steel sheet when forming or press forming the produced steel sheet into the final parts.
Some of methods described in the above patents and/or published patent applications often require strict cooling rate control. The methods often involve several steps of heat treatment and rapid cooling, which are difficult to carry out during commercial production in a steel mill, and thus can restrict the commercial application of these methods. For instance, with respect to mill facility, these extra heating and cooling sections can be prohibitively expensive, and thus, often it is not feasible to add them to many steel mills or hot dip coating lines. Moreover, it is often difficult to maintain good material quality consistently during commercial production, because it is extremely difficult to control the cooling rate precisely during each cooling step when producing steel sheets with various thicknesses and/or widths, as requested by different customers.
Although the existing dual phase steels, in general, exhibit better crashworthiness than other types of high strength steels, a further improvement in impact toughness and crash performance, particularly for thin (i.e., lightweight) steel sheet, is still desired because the requirements and/or regulations for vehicle safety and fuel economy, such as automotive safety and fuel economy, are becoming higher and higher.